Patients who ate a “Mediterranean” diet rich in nuts or extra virgin olive oil as well as vegetables and wine had 30% fewer heart attacks, strokes, or deaths from cardiovascular disease than those that ate a diet that simply lowered their intake of dietary fat. The result, cardiologists say, is likely to change what doctors advise patients who are at risk of cardiovascular disease to eat. The study is being published in the New England Journal of Medicine and presented at the International Congress on Vegetarian Nutrition in Loma Linda, Calif.

Steven Nissen, chairman of the department of medicine at the Cleveland Clinic, called it “an outstanding study with broad societal implications,” noting that the benefit was as big as might be seen with the powerful cholesterol-lowering statin drugs that have become a mainstay of cardiology. He said the result shows “how science can dispel a widely held but fundamentally wrong public opinion”: in this case that an ultra-low-fat diet can make people heart attack proof. Harlan Krumholz, a cardiologist at Yale University and a Forbes contributor, called the result “game-changing.”

“The big issue here is not to cut the total amount of fat in the diet,” says Miguel Martinez-Gonzalez, Professor and Chair of the Department of Preventative Medicine at the University of Navarra and senior author of the study. “If you cut the amount of fat in the diet people will not comply. This is the advantage of the Mediterranean diet. It is high in fat, but it is healthy fat.”

The study carries much more scientific weight than most other studies of diet, which simply examine what it is that healthy people eat. A person who avoids red meat, for instance, might be doing all sorts of other things to remain healthy. What Martinez-Gonzalez and his colleagues did was to test the Mediterranean diet as if it were a drug, in a gold-standard randomized controlled clinical trial.

They recruited 7,447 men and women in Spain between the ages of 55 and 80 who had high blood pressure, high cholesterol, or other risk factors for heart disease but had not had heart problems yet. The researchers randomly assigned these patients to three groups, who were put on three different diets:

A Mediterranean diet high in vegetables, beans and other legumes, fruits, and fish, with at least 4 tablespoons of extra virgin olive oil a day. They were provided with a liter of the oil weekly for the use of their families. If they drank alcohol, they were told to consume at least seven glasses of wine a week. Cookies and other bakery treats, spreads like butter, and processed and red meats were discouraged.

A nearly identical Mediterranean diet that instead of extra-virgin olive oil included walnuts, almonds, and hazelnuts. People were advised to consume 30 grams of nuts a day, which they were given for free.

Patients in the control group were told to avoid fat, including removing any visible fat from food (cooling and scraping it off the top of soup, for instance). They were even to avoid fatty fish. These people received small gifts as part of the study.

The diets in the study. From the New England Journal of Medicine

There were 96 heart attacks, strokes, and cardiovascular deaths among the 2,543 people on the olive oil diet, 83 heart attacks, strokes, and cardiovascular deaths among the 2,454 patients who were on the nut diet, and 109 heart attacks, strokes, and cardiovascular deaths among the 2,450 people on the low-fat diet. When corrected for how long each of those people were on their respective diets, those who were on the nut or olive oil diets were 30% less likely to have a heart attack, stroke, or death related to cardiovascular disease than those on the low-fat diet.

That still means that you would have to put 333 people on a Mediterranean diet for five years in order to prevent a single heart attack, stroke, or cardiovascular death compared to a low fat diet. That's because, in relatively healthy but high risk people, such mishaps are rare. Half the patients were already on ACE inhibitors to reduce their blood pressure and 40% took statins to reduce their cholesterol. But cardiovascular disease is still the leading cause of death. James Stein, a cardiologist at the University of Wisconsin School of Medicine and Public Health, said the study is “a big deal” and that although the absolute reduction in heart attacks and strokes is small, the diet is likely to have other benefits.

The study was not perfect. One big flaw was that at first, the people on the low-fat diet got less counseling than those who were on the diets rich in nuts or olive oil. The researchers realized this was a problem several years in, and changed the study design. Results on how the diets effected cholesterol levels or patients' weight are not yet available.

But there's other supporting evidence. A 2001 study had also shown a benefit for the Mediterranean diet. In 2006, part of the Women's Health Initiative, a group of studies funded by the National Institutes of Health, found cutting fat from women's diets did not prevent heart disease.

That still leaves big questions about exactly what kinds of fat are best. The saturated fats in meat, cheese, and butter are probably bad. The monounsaturated and polyunsaturated fats in vegetables, nuts, and fish are thought to be better. The big question now is whether some of these good fats are even better than others. Are the omega-3 fatty acids in salmon, for instance, better than the monounsaturated fats in olive oil? This study can't answer that question.

What is clear is that there is now a pretty good blueprint for a diet to prevent heart disease. And while the study can really only be directly applied to people who are at high risk, Martinez-Gonzalez has some advice for even healthy, younger people like me: eat like the Mediterraneans do. “This is tasty,” he says, “and this is the future.”